Centralizers are often employed in oilfield and related industries where controlled positioning of a device within a well may be of importance. For example, in the case of a hydrocarbon well there may arise the need to deliver a downhole tool several thousand feet down into the well for performance of an operation thereat. In performing the operation it may be preferable that the tool arrive at the operation site in a circumferentially centered manner (with respect to the diameter of the well). Therefore, a centralizer may be associated with the downhole tool in order to ensure its circumferentially centered delivery to the operation site. This may be especially beneficial where the well is of a horizontal or other configuration presenting a challenge to unaided centralization.
A centralizer may include radially disposed arms biased outwardly from a mandrel or other supporting body in order to contact sides of the well wall, thus, centrally positioning the supporting body. A downhole tool such as that described above may be coupled to the supporting body and thereby circumferentially centered at the operation site. This manner of centralization may be advantageous for a host of different types of operations. In fact, in many operations the vertical alignment of multiple separately delivered downhole tools may be beneficial. In this manner centralization of such tools at an operation site provides a known orientation or positioning of the tools relative to one another. This known orientation may be taken advantage of where the tools are to interact during the course of the operation, for example where one downhole tool may be employed to grab onto and fish out another. Additionally, a host of other operations may benefit from the circumferentially centered positioning of a single downhole tool. Such operations may relate to drilling performance, oil well construction, and the collection of logging information, to name a few.
Unfortunately, the delivery of a downhole tool through the use of a centralizer is prone to inflict damage at the wall of the well by the radially disposed arms of the centralizer. This is because the centralizer is configured with arms reaching an outer diameter capable of stably supporting itself within wider sections of the well. For example, the centralizer may reach a natural outer diameter of about 13 inches for stable positioning within a 12 inch diameter section of a well. However, the centralizer is generally a passive device with arms of a single size that are biased between the support body and the well wall. Therefore, as the diameter of the well becomes smaller the described arms, often of a bow spring configuration, are forced to deform and compress to a smaller diameter as well. For example, the same 12 inch diameter well may become about 3 inches in diameter at some point deeper within the well. This results in a significant amount of compressive force to distribute between the arms and the wall of the narrowing well. That is, as the bowed arms become forced down to a lower profile by the narrowing well wall, more force is exerted thereby on the well wall.
The above described exertion of force can become quite extreme depending on the configuration and dimensions of the arms and the extent of the well's narrowing. As a result, such bow spring arms may prematurely wear out or cause significant damage to the well wall as the centralizer is forced through narrower well sections. This is unfortunate considering that many of these narrower well sections may have no relation to the actual operation site. Thus, the indicated damage may occur in sections of the well where centralization by the centralizer is unnecessary. Furthermore, due to the forces between the centralizer and the well wall, a significant amount of additional force, for example, through coiled tubing advancement, may be required. This may leave coiled tubing, the centralizer, and even the well itself susceptible to damage from application of such greater forces thereupon.
As an alternative to passive centralizers described above, active centralizers such as tractoring mechanisms or other devices capable of interactive or dynamic arm diameter changes may be employed. However, these types of devices are fairly sophisticated and generally require the exercise of operator control over the centralizer's profile throughout the advancement or withdrawal of the device from the well. Thus, such mechanisms are prone to operator error which may lead to well damage exceeding that possible from the above described passive centralizer. Furthermore, rather than reliance on the radially extending natural force of a bowing or similar arm, such devices may require the maintenance of power to the arms at all times in order to attain biasing against the well wall with the arms. Therefore, unlike a passive centralizer, the active centralizer may fail to centralize when faced with a loss of power.